This invention relates generally to transmission devices such as gear mechanisms and, more particularly, to a transmission device suitable for a rotary tool holder of a machine tool used to increase or reduce the speed of a rotary tool for drilling, end milling or cutting.
Ordinary gear mechanisms and planetary gear mechanisms having planetary gears and internal gears are generally utilized as effective transmission devices or speed changing devices in the field of machine tools. Such gear mechanisms are free from sliding during transmission and are therefore advantageous in terms of transmission efficiency. They, however, entail the problem such that the distance between transmission shafts and in turn the pitch circle diameter of each gear often varies due to errors in working the gear and/or elastic deformation of the gear during operation in a loaded condition and, hence, the problem of noise and vibrations thereby caused.
Various types of transmission mechanisms have been proposed with a view to solving these problems, including friction gearing mechanisms, planetary friction transmission mechanisms having friction wheels used in place of the gears of the planetary gear mechanism as disclosed in Japanese Utility Model Post-examination Publication Nos. 56-34194 and 56-42531, and planetary traction transmission mechanisms in which an oil film of a traction oil is interposed between contact portions of the frictional wheels so that power can be transmitted through this oil film. In these mechanisms, the wheels operate in a rolling contact manner, and the power can therefore be smoothly transmitted while noise and vibrations are limited. These mechanisms are therefore suitable for high speed rotation and are advantageous in that the accuracy with which the contact surfaces are worked can be improved easily.
However, since friction gearing mechanisms and traction transmission mechanisms are of the rolling contact power transmission type, they are inferior in torque transmission efficiency as compared with ordinary gear mechanisms and are therefore unsuitable for torque transmission under a heavy load.